In 1991 (BASS v18n1)
I wrote that "the best stereospeaker is one that fulfills the requirements of thehuman auditory system for optimum localization, imaging,and clarity." I felt that an accurate loudspeakerwould likely have a dispersion pattern that would bemore directional than conventional box designs; increasinga speaker's directivity would improve its fidelityat the listening chair. These conclusions cameat the end of a long article on the Carver AmazingMark IV speaker. I pledged to continue my
research into the causes of the "box" sound, and why planarline-source speakers sound different from conventionalcone loudspeakers.

I have more recently concluded that there is no onespeaker type or dispersion pattern that best fulfillsthe requirements of the human auditory system in allplayback environments. For home stereo, however,I believe that the tall planar dipole line-source speakeroffers the best compromise among the importantvariables of imaging, clarity, and envelopment.

Dispersion TypesIn my article, I defined the typical multi-way conespeaker system as having a wide dispersion pattern.In contrast, I mistakenly claimed that the CarverAmazing dipole speaker had a narrower (beamier)dispersion pattern, stating "the narrow dispersionpattern of the line-source driver more closely fulfilledthe auditory requirements of the ear" [the Carver'snarrow ribbon has the wide horizontal radiation thatwould be expected for its size  DRM]. The difficultywith mistakenly applying my 1991 definition of narrowdispersion to the Amazing loudspeaker has comewhen I try to reconcile why I do not prefer speakerswith narrower dispersion even though they typicallyproduce more pinpoint imaging. I am not alone inthis finding. In the December 1997 Audio
review ofthe JBL SVA1600 horn speaker, Don Keele Jr. concludedthat although the imaging and clarity of theJBL were superior to his B&W 801s (a multiway coneloudspeaker), he preferred the "laid-back, staidsound" of the 801s on balance [but also affecting one'spreference would be overall in-room response or tonalbalance and familiarity with a given radiation pattern DRM.]

The Major Comparison
Factors 
Frequency Response and Dispersion Pattern(s)Based on my library and laboratory research, I haveconcluded, as have others, that the best measures ofspeaker quality are frequency response and dispersionpattern.I have not found any credible research showingthat most of the differences we hear among loudspeakerscannot be explained by examining these twovariables. In fact, controlled listening tests have
consistentlyshown that speakers sound the same if they have the same frequency response and dispersion patternand are operated within their linear range. Finally,the speakers must occupy the same space.

The Role of DistortionsMy 1991 conclusion on the minor role distortions ofall types play in determining playback quality stillstands. Harmonic and IM distortion, phase response/time lag, transient response, squarewave reproduction,decay time, etc., measured in my tests and others',have proven to be unreliable indicators of aloudspeaker's playback quality. I cited, among others,the definitive research of Salmi and
Wickstrom,Toole, and at Bose, all of which concluded that suchdistortions pale in significance to frequency responseand dispersion pattern. Tomlinson Holman, duringhis November 1997 Boston-AES/BAS presentation, reportedon German research that similarly concludedthere was no hint of a relationship between such distortionsand perceived playback quality, based on astudy of 45 different speakers in three differentrooms.

But even if not a major factor, distortion is somewhatimportant. My research with multiple listenersindicates that harmonic distortion above 1.2% on 20-60Hz tones is audible, and above 0.3% at higher frequenciesis audible. On complex music, about 10%distortion is considered the requirement for audibility.

In my study with tones, I used two sinewave generators.One fed the main tone while the second generatorwas set to the second harmonic; in otherwords, if 16Hz were under study, the first generatorwas set for 90dBspl at 16Hz and the second was set to32Hz (second harmonic) and its level raised from-100dB (0.001% HD) to a level at which the listener in areal room could detect a difference when the secondtone was switched on or off by a second party,
singleblind.

A regular feature of Keele's reviews in Audio
ismaximum peak power tests. Using his custom tones,he has reported that audible distortion in loudspeakersdoes not occur until extremely high levels arereached. Similarly, Tom Nousaine, in his Stereo
Reviewsubwoofer reviews, has demonstrated that

Wide Dispersion:
Planar Dipole Line-Source
LoudspeakersAn extremely large radiation surface, such as a longribbon, characterizes the planar dipole speaker. TheSound Lab A-1, an electrostatic design, and the WisdomAdrenaline, a ribbon design, are excellent examples.Each is about 6' high. When stereo-only playbackis desired, they and similar speakers have thedispersion pattern most closely fulfilling the auditoryrequirements of the human ear: wide and uniform.

How much direct and indirect energy does such adipole generate? It presents a more diffuse overallsoundfield to the listener because 50% of the energygenerated is projected out the back of the speaker towardthe front wall, away from the listener; thus atleast half of the speaker's output is reflected at leastonce before being heard [in a listening with typicalplacement, though, this is true of all speakers over awide, non-treble frequency range, because of the integratingtime of the ear  DRM]. Because thesoundfield is diffuse in this way, it imparts a greatersense of envelopment  a feeling of being there andof being involved in the music.

How tall does such a speaker have to be to performlike a line source? There are at least two answers tothis question, according to David L. Smith (formerlyof McIntosh, now at Snell) in a 1995 AES conventionpaper. One rule of thumb is that the far field beginsat distances equal to three times the source's largestdimension. In the case of the Wisdom ribbon, thismeans a listener distance greater than 18'. Anotherdefinition of the far field is that point where the linesource's
SPL falls off at the same rate as a pointsource: -6dB with a doubling of distance (the linesourcelevel begins its dropoff with 3dB per doublingof distance) [this may not always be precisely the casein listening rooms  DRM]. At higher frequenciesthe far field is even farther away. Smith concludes,"When long arrays are used for home loudspeakers,the listener is very likely to be in the near field."

When you sit within one foot of any speaker, thedirect sound is much stronger and louder than theroom reflections. This, too, is sometimes referred toas near-field listening. As you move away from
the speaker, you start to hear more of the room. Typically, after about three feet, you hear more of theroom than you do the speaker. In my 1991 Amazingarticle, I quoted Daniel Queen's assertion that a "typicalwide-dispersion loudspeaker permits only about14% of the direct energy to reach the listener."

Power ResponseThe power response of a loudspeaker, the sum ofall the energy radiated from the system, is difficult tomeasure, and this probably accounts for its receivinginsufficient attention in speaker evaluation [plus themania for impulse-based measurement gear  DRM].Ideally to measure power response, one must employan anechoic chamber (or simulate an anechoic environment)plus multiple microphones positioned around the speaker (or a single mike placed at
multiple points), and then sum the total. A comparison ofthe power response with the direct sound defines aspeaker's directivity.

A dipole's bidirectional radiation often means itwill have a flatter power response than a monopoleloudspeaker. Flatness is important because in a roomwe listen chiefly to a speaker's power response, asRoy Allison and some others point out.

A major fault sometimes alleged for dipole speakersis the unnatural' reflection created by the strongrearward radiation toward the front wall [this ischiefly a treble effect compared with conventionalforward-facing speakers, and some find it highlypleasant  DRM]. It arrives at the listener well afterthe initial sound. I maintain that since all speakersgenerate both useful and unwanted reflections withina room, the real questions to settle for the listenershould be: (1) the amount of frequency response alteration,(2) the composition of the delayed sound,i.e., how many early and late reflections are included,and (3) the percentage of direct and indirect sound.

A dipole should be placed at least 7.5' from thefront wall  an adequate distance according to theBBC information provided by Holman during his recentpresentation. Holman stated that a reflection isof negligible importance if it occurs at least 15ms afterthe initial arrival and its energy is at least 15dB lower.Such reflections do not affect either timbre or localization.And longer delays can augment the listeningexperience.

Floor and Ceiling
ReflectionsHaving vertical dispersion restricted means planarspeakers send less energy to the floor and ceiling, soa listener encounters fewer early reflections. Whatfurther distinguishes planars from other designs thataim for partly reduced vertical dispersion, such asmidrange-tweeter-midrange, is that planar speakersmaintain a more consistent response with differenthead heights. In some MTM speaker designs, verticaldispersion is limited only over an octave; above thatrange the speaker is beamy and below it the dispersionis broad.

Boundary augmentation affects planar dipoles likeany speaker, but less so because of the height of thesource driver, its restricted vertical radiation pattern,and the effective multiple distances to the floor andceiling, which distribute the Allison effect over abroader frequency range, tempering its severity. As atest, I placed a cone speaker 18" off the floor, andthere was a dip around 188Hz, just as
Allison's workpredicts. The dip caused noticeable voice coloration, atonal or timbral change that was a clear result of thefloor, front wall, and side wall reflections. To introducea similar 200Hz dip into the output of myAmazing speaker, I used a 1/3-octave equalizer, andthe bottom-of-the-barrel sound that I had associatedexclusively with box speakers was now being exhibitedby the Carvers, pushing the voice from frontstage.

By judicious placement, such boundary-augmentationproblems can be minimized for any design, includingbox speakers, along with other early reflectionsthat color the sound.

There are two primary effects, and one historicalreason, that have instigated the requirement for aseparate center channel speaker in home theaters.Any pair of speakers radiating the same informationcreates a phantom image between them. If onespeaker is louder, or if the listener is closer to onespeaker, this phantom image will shift toward thatspeaker. If the pair of speakers is the left and rightchannels, this shift of the phantom center image willskew, or distort, the front proscenium of sound.Compared with a signal coming only from a singlecenter channel speaker, the interaction of two speakersradiating the same signal causes a frequency responsenotch at around 2kHz at the listener's ears.

This obviously results in a change in timbre.

The movie industry puts dialog in the center channel,since dialog is of primary importance in mostfilms.

As a result of two speakers radiating the same signal,the frequency response balance at the
listener'sears is also gradually boosted in the lower midrangeand bass, due to mutual coupling. Having two speakersradiate the same signal at the same level, midrangeand highs increase 3dB compared with eitherspeaker alone. As the frequency drops and the wavelengthsget longer than twice the distance betweenthe speakers, the coupling gets stronger, ultimatelyreaching +6dB in the bass [this gradual reinforcementis shown in several real-world in-room measurementsgraphed in BASS v17n6
 DRM]. The impact of theseeffects is affected by the reverberant nature of theroom and the speaker dispersion patterns, with

According to Holland and Newell, "Dipole loudspeakers,such as most electrostatics, behave in a differentmanner. The dipole radiation pattern meansthat little or no sound is radiated toward the otherloudspeaker, thus rendering them immune to mutualcoupling effects . Some room-related mutual couplingwill still occur, however, although to a lesserextent than for monopole
loudspeakers."

If tall dipole planar speakers can be so good inthese criteria, why
isn't the design more popular? Thelikely reasons are space limitations, cost, size, visualappearance (spouse-acceptance factor), and the distancerequired from the front wall.

Medium Dispersion: A Cone
LoudspeakerDepending on its size and the frequency range it isasked to reproduce, a cone loudspeaker can have dispersionwider than a planar driver or a narrower directivitythat rivals the horn. As Allison explains it,"Directionality is, with rare exceptions, a function ofthe wavelength of the frequency being generated inrelation to the size of the driver (or the dimension ofthe mouth of the horn) normal to the plane of
interest"[this holds for all drivers, planar as well as cone DRM]. For a 10" woofer, the transition point toless than omni output is about 500Hz and above; for a4" driver it is about 1.4kHz and above; and for a 1"dome tweeter it is the 4-8kHz octave. When driversare called upon to deliver sound higher than thesepoints, their output becomes increasingly concentratedon axis and their off-axis response falls.

Power Response, Reflections
And Horizontal DispersionAllison: "If the power response of the system iswell-dispersed and free of abrupt changes throughoutmost of the audible frequency spectrum, then ourears will interpret the reverberant field as smooth andnatural. [Presuming a relatively flat on-axis frequencyresponse,] if the power response of the system variessignificantly with frequency, we will hear an unevenresponse." With too many cone/box speakers, thepower response of the system falls until the crossovernetwork brings in a smaller driver, at which frequencythe output is again more omnidirectional. Atthis crossover point the dispersion broadens and thepower response jumps up again. Sawtooth power responsecurves like this can easily be heard even whenthe axial output from the system is flat.

A typical two-way cone loudspeaker, such as theParadigm Phantom, has no rear-facing drivers. The
8"and the 3/4" drivers are asked to deliver the entireaudible bandwidth. The result often is a power responsethat does not equal the planar driver insmoothness. James Moir states, "At first thought itwould appear that the reduction in the horizontal offaxisoutput at high frequencies would be of little consequenceto a listener seated on axis, but experienceshows that the effects on sound quality are indeedobvious to a moderately experienced
listener."

The effect of a speaker's distribution of sound is oftendiscussed in audiophile literature, as in commentslike the
"cymbals and trumpets sound betteron horn loudspeakers" or
"they sound too laid-back."What is not often discussed is the cause, or how thespeaker's characteristics  directivity, and frequencyresponse as a function of angle (both of which affectthe ratio of direct and indirect energy as a function offrequency, at the listening position)  are most likelythe cause of the perception.

Since wide and consistent horizontal dispersion isimpossible for a single forward-facing cone driver toproduce, it is better when multiple drivers of differentwidths are used to cover the audio band. Andeven then, both the reflections that influence imaging,and the total in-room power response, sometimeswill be ragged.

Narrower Dispersion: A
Horn-Loaded SpeakerA speaker with narrow dispersion directs moresound forward than to the sides and rear and thus isless affected by the room. This characteristic translatesinto excellent imaging but, of the three majorspeaker dispersion types, with the least sense of envelopmentand spaciousness. Controlled-directivityhorn speakers are known for their clarity and imaging.You can pinpoint the horns; in fact, horns andcymbals sometimes appear to stand out or soundmore forward than the other orchestral instruments.

Some people believe that a stereo loudspeakershould
have a
narrow radiation pattern, like a horn's.It produces less of a reverberant field and some feel itthus is ideal for pop music. It simulates more "theyare
here" than "you are there." It is the opposite of,say, the Bose 901.

The good news is that there typically are fewerearly reflections than from a cone loudspeaker behavior more like that of planar loudspeakers. Thedownside is that a
horn's limited dispersion can meanit is less suited to being used as a lone pair in a stereosystem [depending on your goal and taste  DRM]. Wide-dispersion proponents argue that in any case,since pinpoint imaging is not that important a part ofthe concert experience, it also is not that importantfor playback.

Power Response

The dispersion pattern of a typical horn-loadeddriver, such as the JBL SVA1600, might be quite narrowespecially in the treble, meaning the overall balanceat our ears will probably have too much bass andtoo little highs and will contain the least amount ofreverberant energy [also depending on how close onesits and on the liveliness of the room surfaces DRM]. This imbalance might happen even if theaxis response is flat.

Horizontal DispersionAlthough constant-directivity horns can be designedto have wide and even horizontal dispersion,the equal of [and sometimes better than] many otherspeaker types, most often the radiation pattern is restrictedto a defined listening area, which is great fortheaters. The result is minimized side-wall interferenceand extremely tight imaging  about the best.

Why All This Is Important?The effect of the sound distribution of a loudspeaker its dispersion pattern or patterns  israrely correlated in audiophile writing with what weactually hear in a room.

Correcting my 1991 definition of the narrow-driverplanar dipole speaker to that of a speaker having widedispersion, for example, fits better with the conclusionsreached by the authors listed in that article:Moir, Queen, Kates, et al. According to Moir, "Thesoundfield in a room does not become increasinglydiffuse with the passage of time as is generallythought, but instead becomes increasingly ordered,with the sound energy concentrated in well-definedspatial patterns even at the lower
frequencies." Thus,reverberation is not the decay of a diffuse soundfieldbut the decay of well-defined patterns of energy. Theresulting sound is composed of short and long reflectionsand imperfect frequency response(s). Hence,listening to a narrow-dispersion speaker will be avery different experience from listening to a concert hall experience. [Some listeners feel
not just large-force/large-space classical  DRM.]

Multichannel Sound
RequirementsThere is considerable debate in the home multichannelplayback arena about how many speakers areneeded and what constitutes the ideal dispersion patternwhen the music source is a stereo CD. Signalprocessors have been manufactured to convert existingstereo CD output into surround signals that theirmanufacturers claim provide the best of both worlds:discrete, localized effects that image to the left, center,right, and sides and rear. They also claim to haveeffects that wrap all the way around the listener. TheYamaha DSP-1, the Citation 7.0, and the Lexiconprocessors are among the many units available. Thegoal of these devices is to place the listener in a 3Dsoundfield. To do this most successfully, eachspeaker's dispersion pattern, the number of speakers,and location requirements will be different from
a system set up in accordance with the THX guidelinesfor video soundtracks.

The home speaker setup for the playback of movieswas largely copied from the THX movie theater standards,established after considerable research. However,the playback requirements are not the same ifreproducing music is the main criterion. In the theater,many people sit off-center in a very large room.To keep dialog centered, a center channel was incorporatedin the standard, along with directional frontleft and right speakers. The THX criteria have a frontalbias; the intent is not to enclose you in a musicalsoundfield.

To prevent the listener from localizing sound to theside speakers, dipole speakers were specified. Anadded reason for a diffuse soundfield on the side wasto reduce the audibility of film dropouts, clicks, randomnoises, etc., that enter during the moviemakingprocess, and leakage from the Dolby Surround matrixdecoding of some front-channel sounds.

In the home music system, however, spaciousnessand envelopment are key for many listeners. Stereomeans three-dimensional; only minimum localizationcues are required. The sense of being enclosed orhaving the music all around you requires a differentemphasis, not narrow directionality, especially if youare limited to 5.1 playback channels. According toHolman, for maximum envelopment in a 5.1-channelsystem, the front two loudspeakers should be at ฑ36degrees, the two side channels at ฑ108 degrees, andthe remaining speaker at 180 degrees, behind the listener[the points of a regular pentagon 
DJW].

ConclusionAll speakers in a room generate a total soundfieldthat plays the key role in fidelity. The main concernsshould be to dissect the composition of the sound,my categories being: (a) potentially annoying early
reflections,(b) the more benign late reflections, (c) afrequency response altered by boundary augmentationand then by room dimensions and (d) the proportionsof direct and reflected energy.

The latest studies on the need for envelopment andits causes are right-on. All speakers, whatever theirdispersion, generate a reverberant field in a room,and for maximum high-fidelity envelopment withmusic I submit that we want a soundfield that mostclosely maintains the balance of the information onthe disc [those who feel that most recordings aremade too close to the sound source probably will notwant their playback to be chiefly direct sound, though DRM]. As audiophiles, we have paid too much attentionto reports on the various other distortionsgenerated by loudspeakers. We need more emphasison correlating the speaker's frequency response anddispersion
pattern(s) with what we hear. [And theroom is an equal partner; not even horns can be divorcedfrom the room  DJW.]

This situation can improve if audio reviewerswould categorize speaker system dispersion into mythree main groups of wide, medium, and narrow, andnote dispersion uniformity as a function of frequency.By correctly typing speakers, reviewers will give their readers a better idea of how a given system fits theirboth playback requirements and their environment.